Terahertz waves are electromagnetic waves, having a frequency band component anywhere between 0.03 THz to 30 THz. There are methods to generate terahertz waves using a secondary nonlinear phenomenon by a nonlinear optical crystal. Of these, a technique using electro-optical Cherenkov radiation phenomenon (hereinafter, “Cherenkov radiation”) is capable of generating intense and relatively wide bandwidth terahertz waves, which is described in PTL 1.
Cherenkov radiation is a phenomenon where generated terahertz waves 1302 are conically emitted like shock waves, as illustrated in FIG. 13. Cherenkov radiation occurs in a case where the propagating group velocity of light 1301 propagating through a nonlinear optical crystal is faster than the propagating phase velocity of the terahertz waves 1302. Now, an angle θc between the propagation direction of the generated terahertz waves 1302 and the propagation direction of the light 1301 propagating through the nonlinear optical crystal (hereinafter referred to as “Cherenkov angle”) can be expressed ascos θc=ng/nTHz  (1)where ng is the group refractive index of the nonlinear optical crystal as to light, and nTHz is the refractive index of the medium through which the terahertz waves propagate as to the terahertz waves.
In cases of using such a terahertz wave generating element employing Cherenkov radiation as a terahertz wave generating source in an information acquiring apparatus to acquire information regarding a specimen using terahertz waves, or the like, wavefront shaping may be necessary. PTL 2 discloses a method for second harmonic generation of light propagating through a nonlinear optical crystal, and shaping and externally extracting the generated second harmonic waves using a collimator, as illustrated in FIG. 14.
Second harmonic waves 1405 generated by light 1404 propagating through the nonlinear optical crystal 1402 is emitted in a conical form, and reflected at a reflecting face 1406 of a coupling member 1403. At this time, the wavefront 1408 of the reflected second harmonic waves 1405 is collimated to a high level of planarity. This configuration shapes the wavefront 1408 of second harmonic waves 1405 to be planar, making the second harmonic waves 1405 easier to handle.